Regulating air cushioning and bubble entrapment in charged droplet impact via external electric fields

Air cushioning and bubble entrapment are common phenomena observed during low-velocity droplet impact on solid or fluid surfaces, whereby a thin layer of air mediates the lubrication pressure. Previously, it is found that both charging droplets and the application of external electric fields can eliminate air cushioning and bubble entrapment. In this study, we numerically investigate the air cushioning and bubble entrapment in the charged droplet impacting onto a solid surface under external electric fields. It is found that the presence of net charge has a field enhancement effect on polarized charges on the same side while weakening the electric field of polarized charges on the opposite side, which can be linearly superimposed in the atmospheric environment. Based on the mirror charge model, the scaling law of the electric field threshold required for the impact of charged droplets without air cushioning or bubble entrapment is given. A fitting function based on the scaling law shows a good agreement with the phase diagram of a charged droplet impact modes under electric fields. These findings offer valuable insight for applications that rely on charged droplets' impact under electric fields, such as electrohydrodynamic printing and spray coating, to mitigate the negative impact of air cushioning or bubble entrapment.